1. Field of the Invention
The present invention relates to a chip-type piezoelectric resonator including a capacitor and to a method for adjusting the resonance frequency of the chip-type piezoelectric resonator. More particularly, the present invention relates to a chip-type piezoelectric resonator including a capacitor having a piezoelectric element disposed between dielectric substrates, as well as, to a method for adjusting the resonance frequency thereof.
2. Description of Related Art
A conventional chip-type piezoelectric resonator including a capacitor is shown in FIGS. 7A and 7B.
As shown in FIG. 7A, in a chip-type piezoelectric resonator 51, a first sealing substrate 53 made of an insulating ceramic is laminated on the lower surface of a plate-like piezoelectric element 52, while a second sealing substrate 54 made of a dielectric ceramic is laminated on the upper surface of the piezoelectric element 52.
The piezoelectric element 52 includes a plate-like piezoelectric substrate 52a which is polarized in a thickness direction. As shown in FIG. 7B, a first excitation electrode 52b is disposed on the upper surface of the piezoelectric substrate 52a and a second excitation electrode 52c is disposed on the lower surface of the piezoelectric substrate 52a, such that the electrode 52b and the electrode 52c face each other with the piezoelectric substrate 52a disposed therebetween.
The excitation electrodes 52b and 52c are connected with lead electrodes 52d and 52e, respectively. The lead electrode 52d is disposed on the upper surface of the piezoelectric substrate 52a so as to extend to a peripheral edge thereof, while the lead electrode 52e is disposed on the lower surface of the piezoelectric substrate 52a so as to extend to another peripheral edge thereof opposite to the peripheral edge to which the lead electrode 52d extends.
A cavity 53a is formed in the inner surface of the sealing substrate 53, and a cavity 54a is formed in the inner surface of the sealing substrate 54.
The piezoelectric element 52 is laminated with the first and second sealing substrates 53 and 54 via an unillustrated adhesive layer, to thereby provide a monolithic structure. External electrodes 57, 58 and 59 are disposed on the outer surface of the resulting laminate. The external electrode 57 is connected to the lead electrode 52d to thereby establish electrical connection with the exitation electrode 52b, while the external electrode 59 is connected to the lead electrode 52e to thereby establish electrical connection with the excitation electrode 52c.
The external electrode 58 is connected to ground. Thus, the resonator is connected between the external electrodes 57 and 59 and electrostatic capacitance is provided between the ground and the external electrodes 57 and 59. Since the sealing substrate 54 is made of a dielectric ceramic, the above-described electrostatic capacitance is substantially defined by the sealing substrate 54 between the external electrode 58 and the external electrodes 57 and 59.
In the chip-type resonator 51 shown in FIGS. 7A and 7B, once the sealing substrates 53 and 54 have been bonded to the piezoelectric element 52, the surfaces other than side surfaces, i.e., the upper and the lower surfaces of the piezoelectric element 52, are sealed by the sealing substrates 53 and 54. Therefore, the resonance frequency must be adjusted prior to bonding of the piezoelectric element 52 to the sealing substrates 53 and 54.
However, even after the bonding step of the piezoelectric element 52 and the sealing substrates 53 and 54, the adjusted frequency tends to vary, due to factors occurring in subsequent manufacturing and processing steps. Thus, even if the resonance frequency is controlled with high precision during the step of producing the piezoelectric element 52, the production of a chip-type piezoelectric resonator 51 having high precision in terms of achieving a desired frequency is very difficult. Therefore, the percent ratio of non-defective chip-type piezoelectric resonators 51 is disadvantageously low.
When an external electrode portion of the sealing substrate 54 made of a dielectric ceramic is partially removed after production of the chip-type piezoelectric resonator 51, the electrostatic capacitance changes significantly, so that fine adjustment of the resonance frequency is very difficult.
In addition, since the shapes of the external electrodes as viewed from the upper-surface side is identical to those as viewed from the lower-surface side, automated recognition of the top and reverse sides of the resonator has been difficult.
Another conventional chip-type piezoelectric resonator including a capacitor is shown in FIGS. 8A and 8B.
A chip-type piezoelectric resonator 61 has a package structure including a dielectric substrate 62 and a downwardly-opening cap 63.
As shown in FIGS. 8A and 8B, external electrodes 64, 65 and 66 are disposed on the dielectric substrate 62 such that the electrodes extend from the upper surface of the substrate to the lower surface thereof via two side surfaces. A piezoelectric element 69 is bonded to the external electrodes 64 and 66 via conductive adhesive layers 67 and 68. The piezoelectric element 69 is made of a plate-like piezoelectric substrate 69a. A first excitation electrode 69b is disposed on the upper surface of the piezoelectric substrate 69a, and a second excitation electrode 69c is disposed on the lower surface thereof.
The first excitation electrode 69b is connected to a lead electrode 69d. The lead electrode 69d is arranged on the piezoelectric substrate 69a so as to extend to a peripheral edge thereof and further to extend to the lower surface of the piezoelectric substrate 69a via the side surface. The lead electrode 69d is bonded to the conductive adhesive layer 68 at a portion at which the lead electrode 69d reaches the lower surface of the piezoelectric substrate 69a.
The second excitation electrode 69c is connected to a lead electrode 69e, which is bonded to the conductive adhesive layer 67.
The cap 63, having an opening 63a which covers the above-described piezoelectric element 69, is bonded onto the upper surface of the dielectric substrate 62 via an insulating adhesive which is not shown in FIGS. 8A and 8B.
In the chip-type piezoelectric resonator 61, resonance portions having first and second excitation electrodes 69b and 69c, respectively, are connected between external electrodes 64 and 66. The external electrode 65 is connected to ground. Thus, electrostatic capacitance attributed to the dielectric substrate 62 is provided between the external electrode 65 and each of the external electrodes 64 and 66.
In the chip-type piezoelectric resonator 61 shown in FIGS. 8A and 8B, the resonance frequency can be adjusted, prior to bonding of the cap 63 to the dielectric substrate 62, in a state in which the piezoelectric element 69 has been bonded to the dielectric substrate 62. However, when only the excitation electrode on one surface of the piezoelectric element 69 is machined so as to adjust the resonance frequency, the symmetry between the excitation electrodes 69b and 69c is lost. Thus, spurious vibrations attributed to asymmetry of the excitation electrodes 69b and 69c increases, therefore disadvantageously deteriorating the characteristics of the resonator.
Alternatively, the resonance frequency can be adjusted by modifying the shape of the external electrodes 64, 65 and 66 at the lower surface of the dielectric substrate 62. However, the lower surface of the dielectric substrate 62 is a surface for mounting, and such modification of the external electrodes 64, 65 and 66 causes variation of mounting conditions among individual chip-type piezoelectric resonators 61.